1. Field of the Invention
The present invention relates generally to lightning detection systems, and more particularly, to lightning detection systems utilizing a gated magnetic direction finder and waveform analyzing circuitry for rendering the magnetic direction finder operative to measure the direction of the lightning discharge for only the initial few microseconds of the discharge, and only after it has been determined from an analysis of the detected waveform that a ground stroke has occurred.
2. Description of the Prior Art
In many applications, it is often necessary to determine accurate locations of lightning discharges to ground. For example, accurate knowledge of lightning discharges aids in the precise location of forest fires and power system damage, and is useful in providing an indication of the path of an approaching thunderstorm so that recreational areas, such as golf courses and swimming pools in the path of the storm may be safely evacuated.
Several systems for locating lightning discharges are known. Among such systems are systems that utilize a magnetic cathode ray direction finder employing a pair of orthogonal loop antennas tuned to a VLF frequency, typically 10 kHz, for detecting the horizontal magnetic field produced by the lightning discharge. Such systems range from simple magnetic direction finders wherein the outputs of the orthogonal antennas are simultaneously applied to the X and Y inputs of an X-Y oscilloscope to generate a vector display indicating the direction of the lightning to more sophisticated systems that use gating and waveform analysis techniques that permit the vector to be displayed only when the detected field has certain predetermined characteristics indicative of a ground stroke. A lightning detection system using gating and waveform analysis techniques is described in U.S. patent application Ser. No. 732,365, filed Oct. 14, 1976, now Pat. No. 4,115,732 and incorporated herein by reference.
While these systems do provide a way to detect lightning discharges, the accuracy of conventional ungated cathode ray detectors is poor at distances of less than about 200 kilometers because the magnetic field sensing antennas sense the magnetic field generated by the entire discharge, including the field generated by horizontally and diagonally extending branches, as well as ionospheric reflections. Such effects result in bearing errors in excess of 20.degree. at distances of about 200 kilometers, and substantially larger errors for distances of less than 20 kilometers. The system disclosed in the above-mentioned application Ser. No. 732,365 and now U.S. Pat. No. 4,115,732 uses waveform analysis and gating techniques in an attempt to reduce the effects of magnetic fields generated by sources other than the main vertical channel of the stroke extending from the ground. The gating and waveform analysis results in a substantial increase in accuracy; however, the system is not designed to sense the polarity of the field generated by the stroke, and thus resulting in a 180.degree. ambiguity to some strokes. Furthermore, the system is not designed to have different waveform analyzing criteria for an initial stroke and subsequent strokes. Also, the system has some difficulty in rejecting certain types of invalid lightning signals and some difficulty is encountered in measuring the exact width of a lightning signal.